Contact lenses are divided into two main classes: hard lenses and soft lenses, or so called "hydrogel" lenses. Each of these has advantages and disadvantages. The main advantage of a hard lens, such as the one made for instance from cellulose acetate butyrate (CAB) or from polymethyl-methacrylate (PMMA) is its mechanical strength and dimensional stability. A disadvantage is that they are rather uncomfortable to wear. A soft lens made of water swellable polymers, is more comfortable to wear, but becomes oxygen permeable only at a rather high water content which, however, also causes a drastic decrease in its mechanical strength (tear strength).
Both these types can be manufactured by various methods, one important method comprising the following steps:
(a) The material is first prepared in the form of a solid rod and cut up into small discs.
(b) These discs are then machined on a lathe to basically predetermined dimensions, both on the convex and concave surfaces.
(c) Both surfaces are then polished to the final prescribed dimensions.
(d) In the event of a hydrogel type lens, the finally machined and polished lens is equilibrated with a saline solution of a specified salt concentration whereby it assumes its final shape, being ready to wear. On the other hand, the hard lens usually absorbs only very minor amounts of water and is ready to wear following the polishing operation.
While these two types dominate the market of contact lenses at the present time, there exist several variations of these two classes. For example, a hard lens may be made more comfortable to wear by grafting onto its surface a very thin film of of a hydrophilic monomer or otherwise by hydrolyzing its surface. In this way, the advantages of the hard lenses are combined with the tissue tolerance of a hydrogel-type lens.
According to conventional practice of contact lens manufacture, the lens is mechanically machined (cut and polished) starting with a fully polymerized material in the form of a small cylinder. After having machined and polished one of the lens surfaces, the semi-finished lens is attached to a tool, usually by means of a low-melting wax, and carefully centered so that its axis coincides exactly with that of the lathe. Any misalignment of the lens and the machine axis will cause an optical fault in the lens. This is a laborious and time consuming step in the manufacture of the lens.
This step is reportedly eliminated by using a technique known as "centrifugal casting" or by employing matched die molding methods. According to these technologies, the two axes in a lens are automatically coincident, and no misalignment can take place.
These methods require costly tooling, which is justified only for large scale production. These methods are not generally applicable to the various materials that are being used in lens manufacture. For example, the matched die molding technique may be used only with thermoplastic resins, such as CAB (Cellulose Acetate Butyrate), but not with cross-linked polymers, such as the soft lenses made of crosslinked polyHEMA.
Another method is described in U.S. Pat. No. 4,155,962. According to this method, a thermosetting casting resin is cast into a cup-like mold, preformed by means of a master mold, from a thermoplastic resin by injection molding. It is claimed that the posterior surface of the lens is formed by replicating the surface of the plastic cup-like mold. However, it is clear that the plastic cup itself cannot be exactly multiplicated by an injection molding process, due to shrinkage of the plastic material in the cooling step which is of a stochastic, uncontrollable nature. For producing a lens surface of optical quality these dimensional variations between the injection molded cups of said invention, cannot be tolerated.
Since a surface made of a thermoplastic resin is not inert towards a casting resin, the interaction of the casting resin with the preformed plastic cup-like mold may be of two opposing kinds: if it is slightly swelled by the casting resin--it will stick to the finished posterior surface of the semi-finished lens. This may be defined as a positive interaction. To eliminate such a possibility, said patent suggests the use of parting agents with which the cup-like mold should be coated prior to the casting operation. This not only introduces an additional operation, but also causes certain surface distortions, detrimental to the optical quality of the ultimate posterior lens surface.
In case of a "negative" interaction of the casting resin with the surface of the cup-like mold of said invention, which manifests itself by non-wetting of said surface, as for instance when a Teflon .TM. fluorocarbon resin cup is being used, the subsequent machining of the lens becomes impossible, since the slightest force of the cutting tool will cause separation between the molded lens precursor and the said cup.
U.S. Pat. No. 4,247,492 describes another embodiment of the plastic cup lens casting process, whereby the thermoplastic resin is subjected--during the injection molding of the cup--to high shear forces in order "to break-up macromolecular chains", thereby producing active species at the surface of the cup in order to improve the adhesion of the lens precursor to the cup surface, so as to avoid premature demolding of said lens precursor.
It is clear from these contradicting approaches to the lens-casting method in a plastic disposable cup-like mold, that the casting method is complicated and inefficient and can hardly be a viable alternative to conventional lens forming technology.